1. Field of the Invention
This invention pertains to optical supercontinuum generation or broadening of bandwidth of an optical signal whereby wavelength of the signal is broadened between about 2 and 14 microns.
2. Description of Related Prior Art
In supercontinuum generation, pulses of femtoseconds (fs) to nanoseconds (ns) are spectrally broadened by various nonlinear processes, including self phase modulation, stimulated Raman scattering and four wave mixing, dependent on the pump temporal properties and the dispersion slope of the fiber to create a light continuum much broader in wavelength than the pump bandwidth. While supercontinuum generation is possible by focusing a high intensity light into a nonlinear medium, much broader bandwidths and significantly lower thresholds are possible when the pump is focused into an optical fiber where the guiding characteristics of the fiber allow long pump interactions with the nonlinearities of the fiber materials.
Supercontinuum generation is possible in normal optical fiber or dispersion shifted fiber, however, to achieve maximum supercontinuum bandwidth and lowest supercontinuum threshold, the pump wavelength must be near the zero dispersion point or in the anomalous despersion region of the fiber. The use of photonic crystal fiber meets these criteria and allows lower thresholds and greater bandwidths with proper fiber design.
Photonic crystal fiber is an optical fiber whose guiding solid core region is surrounded by air holes. The air holes create a reduced index cladding which guides light in the solid core region. The advantage of photonic crystal fiber over conventional core/clad fiber is that the dispersion of the fiber can be easily tailored by manipulating cladding microstructured hole size and periodicity as well as core size. In addition, very small core sizes are possible leading to high power densities and resulting in increased nonlinearities in the fiber. For supercontinuum generation, this allows the photonic crystal fiber to be tailored to the pump to maximize the supercontinuum generation and minimize the threshold.
Supercontinuum generation has been demonstrated in silica photonic crystal fiber in the visible and near infrared. In silica photonic crystal fiber, however, the multiphonon edge of silica glass limits the transmission window in the infrared. This, in turn, limits the extent of spectral broadening in the infrared so that, to date, supercontinuum spectra have only been able to span from about 400 nm to about 2.2 μm.
Many applications exist for bright broadband infrared sources beyond about 2 μm. Of particular interest are light sources in the chemical and biological “fingerprint region” from 3–12 μm for biological and chemical sensing and sources within the atmospheric transmission windows from 2–5 μm and 8–12 μm for infrared countermeasures and certain radar (LIDAR) applications. Other applications for such sources include infrared illuminators and infrared sources for hardware-in-the-loop testing. Supercontinuum sources in the infrared would enable these applications.
Unfortunately, transmission limitations of the silica glass matrix of silica limits the supercontinuum generation to less than about 2 μm. For supercontinuum generation in the infrared, alternate technologies and materials are needed.
As disclosed herein, it has been demonstrated that supercontinuum generation in the infrared is possible in chalcogenide based fiber. Chalcogenide glass is highly transmissive in the infrared and thus spectral broadening is not limited by the multiphonon edge, as in silica. Also, chalcogenide glass has much higher nonlinearities that silica glass and thus efficient supercontinuum generation is achievable with lower peak powers and shorter fiber lengths than silica. These high nonlinearities of chalcogenide glass even allow efficient supercontinuum generation with continuous wave (CW) laser sources. Nonlinearity of silica is about 2.5×10−20 m/W whereas nonlinearity of a chalcogenide is at least 100 times higher.